1. Field of the Invention
The present invention relates to a display device and, more particularly, to a display device having an optical film on the front surface of its display screen.
2. Description of the Related Art
Since a display screen of a display device is generally a glass screen, it causes a phenomenon in which clearness of an image formed from inside the display device is degraded by primary reflection of ambient light from the display screen or the like.
For example, since a faceplate outer surface of a cathode-ray tube is normally a smooth glass surface, if light from an ambient light source is primarily reflected by the faceplate outer surface, an image formed from inside the faceplate overlaps the reflected light to largely degrade its clearness.
Means for solving the above problem are roughly classified into two means. The first means is a method of scattering ambient light by forming fine projections and recesses on the faceplate outer surface, as disclosed in Published Unexamined Japanese Patent Application No. 61-29051. In this method, however, the entire screen is whitened to decrease the contrast of an image. In addition, the resolution of an image is also decreased. The second means is a method of preventing reflection by forming a single or multilayered optical film on the faceplate outer surface. This method can provide a satisfactory strength and a non-reflection condition without largely decreasing the contrast and resolution of an image. However, it is difficult to easily form an optical film.
That is, in the case of a single layer, such an optical film is formed of a substance having a refractive index lower than that of glass constituting the display screen. The reflection condition is represented by n.sub.1.sup.2 =n.sub.0 n.sub.2 where n.sub.1 is the refractive index of a thin layer, n.sub.0 is the refractive index of air, and n.sub.2 is the refractive index of the display screen of the display device.
For example, the display screen of a cathode-ray tube consists of glass, and its refractive index n.sub.2 is about 1.52. Since the refractive index n.sub.0 of air is 1.00, the refractive index n.sub.1 of an optical film which satisfies the non-reflection condition is preferably 1.23. In general, however, almost no substance having such a refractive index is known. For this reason, it is difficult to obtain an optical film which satisfies the non-reflection condition with a single layer.
If the antireflection film has a two-layered structure, however, the non-reflection condition is n.sub.0 n.sub.2 =n.sub.1 n.sub.3 where n.sub.0 is the refractive index of a substrate of the display screen, n.sub.1 is the refractive index of a lower layer, n.sub.2 is the refractive index of an upper film, and n.sub.3 is the refractive index of air. In the case of a cathode-ray tube, a value of n.sub.1 /n.sub.2 =1.23 is obtained by substituting for n.sub.0 and n.sub.3, 1.52, the refractive index of glass, and 1.00, of the refractive index of air, respectively. That is, in order to satisfy the non-reflection condition, it is ideal to form an optical film having a two-layered structure in which a refractive index ratio of the lower to upper layer is 1.23 on the faceplate outer surface of a cathode-ray tube.
A vapor deposition method or a sputtering method is well known as a method of forming such an optical film. To apply these methods to a comparatively large substrate such as a display screen of a display device, however, is not practical since they require a comparatively large system. In order to form an optical film on such a large substrate, therefore, a method of forming a film by coating a solution containing a film formation material on a substrate of a display screen and drying the solution is much easier than the above methods. As such a formation method, a so-called solgel method, is often used. In this method, a metal alkoxide, for example, is used as a starting material and subjected to hydrolysis and dehydrating condensation to obtain a metal compound as a condensate.
Published Unexamined Japanese Patent Application No. 61-10043 discloses a method in which a lower layer consisting of a co-condensate of an oxide-based compound of Ti or Zr such as an alkoxide of Ti or Zr and an SiO.sub.2 -based compound such as an alkoxide of Si is formed on a glass substrate, and an upper layer consisting of a fluorine-containing silicon compound obtained from alkoxysilane or chlorosilane containing a polyfluoroalkyl group is formed on the lower layer, thereby obtaining an antireflection film which satisfies the non-reflection condition. The co-condensate constituting the lower layer is obtained by two processes of hydrolysis and dehydrating condensation of a metal compound. If an alkoxide is used as the metal compound, the two processes are represented by the following reaction formulas: EQU Hydrolysis: M(OR).sub.n +xH.sub.2 O.fwdarw.M(OH).sub.x (OR).sub.n-x +xROH EQU Dehydrating condensation: M--OH+HO--M.fwdarw.M--O--M+H.sub.2 O
wherein each of n and x represents a natural number, M represents a metal element, and R represents an alkyl group.
The hydrolysis rate of a metal alkoxide largely changes in accordance with the type of a metal. For example, the hydrolysis rate of an Si alkoxide is lower than that of a Ti or Zr alkoxide. When an oxide of Si and Ti or Si and Zr is to be formed, therefore, since a Ti or Zr alkoxide is selectively hydrolyzed earlier in the starting solution, the composition of a formed thin layer becomes non-homogeneous. For this reason, no satisfactory strength can be obtained by a film formed by the invention described in Published Unexamined Japanese Patent Application No. 61-10043.
As described above, in the conventional display devices, if fine projections and recesses are formed on a display screen of the device in order to prevent primary reflection on the display screen, the contrast or resolution of an image is degraded. In addition, if an antireflection film is to be formed by a single layer, a substance having a refractive index which satisfies the non-reflection condition is not available. Even if a two-layered structure is adopted, a film having satisfactory strength and capable of being easily formed cannot be obtained.